The identifier used is "smashwords". It will show in the details as "Smashwords".

Special Notes:

It retrieves the full size covers that are displayed when clicking on the cover when viewing the book. The usually have a height of 2200px.

Smashwords shows a category. This can have multiple values and levels. I haven't decided what to do with this yet.

For the series, Smashwords has both an index and what they call "custom_membership_statement". The latter seems to be descriptive text for where the book in the series. As an example, one book had this as "Volume 0.5", but the series index was "1". The plugin attempts to get the series index from the "custom_membership_statement".